Hi Paullo. There are a few things that might be useful to know regarding mezzanine.mobile. First off, "vanilla" Mezzanine (without mezzanine.mobile) is responsive because it's based on Bootstrap 3, which provides a wide variety of responsive utilities. This means you will get a mobile menu and page layout without activating mezzanine.mobile.
So, what's the use of mezzanine.mobile, you may ask. Well, mezzanine.mobile allows the server to respond to mobile users with custom templates (or any other custom logic, for that matter). The difference is that mezzanine.mobile serves completely different and independent HTML templates, while Bootstrap takes your standard HTML template and adapts it to mobile devices with CSS and Javascript.
Depending on your application, you will want to go with one approach or the other. My advice for newcomers: leave mezzanine.mobile off and use Bootstrap for your responsive needs, it will suffice most of the time :)